Ionization Constant of Weak Acid

 

PURPOSE

 

            The purpose of this experiment is to determine the ionization constant of an organic acid, or indicator.  The techniques to be used include using a pH meter and spectrophotometry.

 

DISCUSSION

 

            An acid-base indicator is a weak acid and ionizes according to the equation:

 

 

The ionization constant may be expresses as:

 

 

If we let x represent the fraction of indicator in the ionized form, then:

 

 

Thus we may use this expression for K.

 

or

 

            pH can be determined using a pH meter.  The ratio  can be determined using a spectrophotometer and Beer’s Law, which states that absorbance is proportional to concentration.

 

            In an acid solution most of the indicator will be in the form HIn.  In a basic solution nearly all of the indicator will be in the form In-.  At intermediate pH there will be some HIn and some In-.

 

            If we choose a wavelength where either HIn or In- absorbs strongly the total absorbance will be made up of absorbances contributed from each form.

 

If we let Aa represent the absorbance in the most acid solution and Ab the absorbance in the most basic solution, then at intermediate pH the absorbance is:

 

 

Solving for x we find:

 

 

If we substitute this into the equation for pK, we obtain:

 

 

If we plot vs.  pH  we should obtain a straight line.

 

At the intercept where is zero, the pH is pK.

 

Also, where pH = 0,          

  

 


EQUIPMENT AND CHEMICALS

 

            Spectrohotometer (visible, such as Spectronic 20, Turner 350, or Coleman 124)

            pH meter

            1 M HCl, small amount

            1 M Sodium Acetate, 50 ml

 

            Indicator stock solution to be made up as follows:

                        (a)  approx. one-half gram indicator

                        (b)  15 ml  0.1 M NaOH

                        (c)  Sufficient distilled water to make one liter.

 

            Indicator may be methyl orange, methyl red, bromphenal blue, or bromcresol green.

 

PROCEDURE

 

            Add 2 ml  indicator to 50 ml sodium acetate in a 250 ml volumetric flask.  Fill to the mark with distilled water.

 

            Find the absorbance of this solution in the range 350 - 650 nm. Use a recording spectrophotometer, such as Coleman 124 or P. E. Lambda 3.

 

            Determine the pH of the solution.  (Be sure the pH meter is calibrated!)

 

            Add a small amount of 1M HCl to reduce pH and again determine absorbance as a function of wavelength.  Measure the pH of the solution.

 

            Repeat the procedure over the range pH 2-6 for a total of at least five absorption curves.  When using Coleman 124 all runs may be made using same recorder paper.

 

            At a wavelength corresponding to one of the absorption maxima, plot against pH.  The graph should be linear.  At either intercept, determine pK.  Note:  The number of points on your graph will be the number of runs minus 2.